The poverty rate of children in the US, last I saw, is about 26%. My question is -
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"What's the long term economic impact of poor children not being able to receive adequate healthcare (as well as nutrition) during their formative years, including grades K-12?"

Great post, and shows that, as much as we like to think we know how to develop economies, we still don't know an awful lot.
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This info comes on the heels of another recent study which showed that the best way to help poor, rural folks in Kenya was to just give them money, about three years' worth of income, no strings attached.
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That flew in the face of the "teach a man to fish" doctrine, and put a lot of development work on it's head.

'Poor people also buy too little preventive health care for themselves, even though the benefits are huge.' Prevention involves investment in health care today which increases the chances of better health outcomes in the future. A poor person who already has to prioritise spending for his or her immediate needs may well make a rational decision against such investment with uncertain future benefits. To be effective these investments cannot be left to the individual. This is true for universal health insurance in the US today (affordable care act) as it was in Rome more than 2000 years ago, where sanitation was a state affair ('cloaca maxima').

I responded to your comment: 26% of the country's children are in poverty, and the educational and economic outcomes of children in poverty in the US are poor.
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So, your 5% comment is either incorrect or irrelevant, and I haven't seen you add info beyond personal opinion.

Taken further, it sounds like you're trying to use poverty indicators - some of which have been constructed in the course of public policy - as an ad-hominem argument to diminish the suffering of those in poverty.
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And, taken to it's furthest conclusion, that's like saying that 0 degrees Fahrenheit and 100 degrees Fahrenheit cannot be differentiated by those experiencing them, because they're both heat indicators sanctioned by government, and government is inherently corrupt (in your view), or something.
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As I mentioned previously, TE comment sections are known for thinking at a higher level than what you've exhibited thus far. Please try harder.

Why would you make a statement alleging that allowing children to suffer may lead to positive outcomes, in a data-driven publication like TE, without the data to actually support it? This ain't Yahoo News.

"The government's 'poverty rate' is a made up statistic..."
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Do you have a source for this, or are you putting forth a conspiracy theory?
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"I make no claim whatsoever as to whether 'raising children in poverty leads to positive economic outcomes.'"
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Actually, what you wrote above was that making children suffer now would most likely, in your view, lead to fewer children suffering later. Do you actually have any data to support this view?

"...the definition was made up by the people who readily admit to doing so."
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Okay. I think what you're trying to say is that, given that the public sector created poverty indicators for that particular website (although the private sector has created such indicators as well) -
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Then the indicators are therefore arbitrary (I think that's what you're saying) -
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And therefore poverty in the US doesn't exist. Or something.
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Is that what you're trying to say?

Wow. Just wow. My heartfelt congratulations to the economists who adapted RCTs to economics. To think, it took until this century to do that. Maybe it took that long to get the 19th century ideologies out of our systems enough to start thinking.

"Yes, the definition of poverty created by government bureaucrats..."
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Ad-hominem, and a logical fallacy.
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"..is arbitrary, as the OMB's Statistical Policy Directive No. 14 demonstrates."
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The OMB does not claim that the definition is arbitrary. You're calling it arbitrary, and have thus far not provided evidence beyond an irrelevant website of stats to support your claim.
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"...would seem to indicate you are not taking this conversation seriously."
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Unnecessary personal attack. If you are having difficulty making your case, I'd suggest that you not blame your audience.
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It's apparent we're not going to agree on your "point number one". I suggest we move on to point number two, where you suggested that children and suffering could provide positive outcomes. I'm looking forward to see how you try to justify that particular comment.